Two-way telephone and like electrical system



Feb. 26, 1935. G, MURPHY 1,992,833

TWO-WAY TELEPHONE AND LIKE ELECTRICAL SYSTEM Filed Sept. 21, 1933 4 Sheets-Sheet 1 INVENTOR 'g' r Ema/ WW BY ATTORN EY Feb. 26, 1935. F. M. e. MURPHY TWO WAY TELEPHONE AND LIKE ELECTRICAL SYSTEM 4 Sheets-Sheet 2 Filed Sept. 21, 1953 INVENTOR F. M.6. MURPHY Aim ATTORNEY uvwi Tut E Q F 1935. F. M. G. MURPHY TWO-WAY TELEPHONE AND LIKE ELECTRICAL SYSTEM Filed Sept. 21, 1933 4 Sheets-Sheet 5 1935- F. M. G. MURPHY TWO-WAY TELEPHONE AND LIKE ELECTRICAL SYSTEM Filed Sept. 21, 1955 4 Sheets-Sheet 4 INVENTORY F.M.G M URPHY N a Q ATTORNEY Patented Feb. 26, 1935 UNITED STATES "PATENT OFFICE TWO-WAY TELEPHONE AND LIKE ELECTRICAL SYSTEM Francis Money Graham Murphy, Chelmsford, England, assigncr to Radio Corporation of America, a corporation of Delaware Application September 21,1933, Serial No. 699,320 In Great Britain September 3, 1932 4: Claims.

This invention relates to two-way telephone and like electrical signalling systems and has for its object to provide a two-way system suitable line through the usual hybridcoil set. As is well known, in such an installation an ideal hybrid coil set would be one in whichthe balance was so perfect that there would be no interaction between the receiving and transmitting channels while the loss between either of these channels and the subscribers line was a minimum. Accurate balance in a hybrid coil set involves that the impedance of the subscribers line shall be accurately simulated at all frequencies employed, and since, in the majority of practical cases,the subscribers line is apt to be changed from time to time (c. g. in the case of a radio telephoneservice the subscribers line in use at any time may be any of a plurality of lines'leading to differently situated subscribers) .thesaid simulation cannot be more than approximate and therefore some out-of-balance will in practice almost always exist. For these reasons it has been-customary to employ so-called blocking devices in connection with'the transmitting and receiving channels so that both channels cannot be in use at the same time. Such blocking devices, which are controlled in operation by the speech or other signal currents, may take any of a variety of different forms one of the most usual arrangements being that in which normally (i. e. in the absence of speech) the transmitting channel is paralyzed (i. e. inoperative) and the receiving channel made (i. e. operative) speech currents coming from the subscribers line being utilized to .make the transmitting channel'and paralyze the receiving channel. Obviously, the device which paralyses the one channel and makes'the other, must not operate when no speech is pass- It is also known so to arrange matters that the operation of the blocking device is dependent, not merely upon the energy in the transmitting channel, but differentially upon this energy and upon the energy in the. receiving channel; Now

it will be appreciated that there will be a more or less substantial noise level in the receiving channel and this fact combined with the fact that perfect balance is in practice not obtained in the hybrid coil set, introduces a diificulty. The fact that the output from the receiving amplifier is generally required to be large and the input to the transmitting amplifier maybe quite small, introduces difliculty since the difference between the energy level at the input'terminals 1 of the transmitting amplifier due to noise passed through the hybrid coil set, and the energy level at the same terminals when there is speech to be transmitted, is not very great; and, in fact, not only is the margin for operation of the blocking device not very great, but 'the necessary adjustment for satisfactory operation is shows, diagrammatically, by way of example, a 3

well known type of two-way: telephone'system; Figure 2 illustrates a modification of the system of Figure 1 in accordance with the principles of the present invention; Figure 3 illustrates in some detail a desirable feature of the invention; and Figure 4, which is shown in parts 4a, 4b, 4c, illustrates a practical embodiment of the invention.

In orderthat the aforementioned difficulties may be the better understood a typical well known installation will now be briefly described.

In this known installation which is represented schematicallyin the accompanying Figure 1 the subscribers line L is associated through the usual hybrid coil set H with a transmitting channel and with a receiving channel. Included in the transmitting channel is a switch or contact device D whereby said channel may be opened or closed i. e. disconnected from or connected to the radio or other transmitter proper, while the receiving channel leads from the radio or other receiver proper may be connected through a switch or contact device E and then through the receiving amplifier G to the hybrid coil set. A blocking amplifier detector unit A is connected to receive its in- 4 input is energized from a point between the input terminals of the receiving amplifier G and the contact device E in the receiving channel.

Now with this arrangement suppose that when no speech is passing,.the noise level at the input terminals of the receiving amplifier G is r and that the gain of the receiving amplifier may be expressed by a power ratio g. Suppose also that the loss in energy in passing from the receiving channel side of the hybrid coil set H through the said set to the transmitting channel may be expressed by a power ratio It and that when speech to be transmitted comes from the subscribers line the energy thereof may be expressed by the quantity t. Now since speech from the subscribers line will cause equal E. M. F.s in both the transmitting, and receiving channels but no E. M. F.

across the balancing network of the hybrid coil set H and since, moreover, speech passing from the receiving channel towards the subscribers line will cause equal E. M. F.s across the said line and across the balancing network when no speech is passing, the input energy to the blocking amplifier detector may be expressed by r g h' (ignoring the noise from the subscribers line).

When speech from the subscribers line arrives this amplifier detector will receive an energy level r g h+t. It will therefore be apparent that the margin for operation of the blocking device is small, being the amount t and that the adjustment of the blocking amplifier detector A and of the gain controlling amplifier detector B must be very accurately made.

The principal object of the present invention is to render the necessary adjustments less critical and this object is achieved according to the main feature thereof by taking one of the energies which are to be difierentially utilized for the control of the blocking device from the receiving channel side of the hybrid coil set instead of from the transmitting channel side thereof.

In one Way of carrying out the present invention the known arrangement hereinbefore described is modified as shown in the accompanying Figure 2 by connecting the input terminals of the blocking amplifier detector A to the receiving channel immediately adjacent the hybrid coil set instead of connecting these terminals to the transmitting channel adjacent the hybrid coil set.

Now with this arrangement, when no speech is passing, the energy level at the input terminals of the blocking amplifier detector will be r X g and, when speech from the subscribers line arrives, the energy level at the said input terminals will be r +t. The margin for operation is therefore still i but the necessary adjustment is less critical since it is now only necessary to adjust for a balance of the effects of TX g and r. The practical importance of this improvement will be realized from the fact that when only the impedance of the subscribers line is changed, e. g. when the hybrid coil set is changed over from one subscriber to another both the value of h is changed and the value of the impedance facing the output terminals of the receiving magnifier is changed, i. e. the apparent gain of the receiving amplifier is changed. These two effects are mathematically related in a manner involving the simulating or balance network of the hybrid coil set, the impedance of the subscribers line, the hybrid coils, and the impedances of the transmitting and receiving channels at the appreciate hybrid coil set terminals. In practice the value ,of h is subject to large changes whereas the apparent change in the gain of the receiving ampliin the accompanying Figure 3.

fier is relatively small and therefore the arrangement of the present invention provides a practical and important improvement inasmuch as the adjustments necessary for satisfactory operation are rendered substantially less critical.

A second feature of the invention is concerned with the means for differentially utilizing the two energies for control of the blocking device and, according to the said second feature, instead of employing a gain controlling amplifier detector to control the gain of the blocking amplifier detector the outputs from two correspondingly energizedamplifier detectors are differentially combined and the combined energy utilized to control the blocking device.

One arrangement in accordance with this feature of the invention is illustrated schematically In this figure the output from an amplifier A--whose input terminals are energized either (and preferably) from the receiving channel side of the hybrid coil set (as is the device A of Figure 2) or from the other side of the hybrid coil set (as is the device A of Figure l)is passed through a transformer T1 one of whose secondary terminals is connected to the grid and anode of a triode V1 the other secondary terminal being connected'through a time control circuit, consisting of a resistance R1 and condenser C1 in shunt, to the cathode'of the triode. Similarly the output terminals of an amplifier B whose input terminals are connected to the input terminals of the receiving amplifier are connected to the primary of a second transformer T2 whose secondary terminals are associated with a second triode V2 and a time control circuit C2, B2 in the same way as are the secondary terminals of the first transformer. The cathodes of the two triodes are connected together and connections are taken from the grid and cathode respectively of a third triode V3 to the ends (remote from the cathodes) of the re sistances R1 and R2. The anode and cathode of the triode Vx'are connected together through the usual anode battery HT in series with a resistance R3 shunting a condenser C3.

It will be seen that with this arrangement if the voltages passed through the two transformers T1, T2 are equal the voltages across the two time control circuits C1, R1 and C2,'R2 will be equal. The two triodes V1 and V2 of course act as diode rectifiers. In these circumstances, therefore, the same voltage will be applied both to the grid and to the cathode of the triode V3 and so long as the voltages through the two transformers are the same there will be no change in the voltage across the resistance R3 in the plate circuit of the third triode. If, however, the voltages through the trasformers become different a differential action occurs and there will be a change in the voltage across the resistance R3.

This voltage is utilized in any convenient way (and after direct current amplificaton if required) to control the blocking device C3. Preferably the connections between the grid and cathode of third triode and the appropriate resistances in the time control circuits are made, not as above stated and as shown to the ends of these resistance, but to adjustable tapping points thereon the condensers in the two time control circuits being connected between the appropriate cathodes and the appropriate tapping points. Preferably also the time constants of the two circuits which have been above described as time control circuits are much smaller than the time constant which will, of coursabe given by the resistance-condenser combination C3, R3 in the plate circuit of the triode V3. This arrangement produces the result that when the output from theamplifier A becomes greater than that from the amplifier B, the voltage across the resistance R3 is progressively reduced to zero and cannot become reversed; i. e. the voltage is limited and the time of restoration to its initial value will become independent of any further increase in the excess in the output of one amplifier over that of the other so that what may be termed the hang over is rendered inde pendent of the input. This desirable result would not be achieved if the time constants of the circuits associated with the rectifying triodes V1, V2 were high for the time of restoration in such a case would depend upon the voltage across these control circuits.

The control circuits can, if desired, be arranged to be adjustable as to their time constants.

Figure 4 of the accompanying drawings, which is shown in three parts, namely 4a, 4.2), and do, illustrates in some detail a practical embodiment of the invention, said embodiment being in sheet an arrangement of the type illustrated in Figure l, but incorporating the principal feature of the arrangement shown in Figure 3.

In the arrangement of Figure A noactual mechanical switches are employed, the switching effect of the devices D and E of Figure 1 being obtained purely electrically by rendering certain thermionic valves operative or inoperative by alteration of the grid biasses thereof.

Referring to Figure 4a a radio receiver (not shown) is connected through a suitable filter illustrated within the dotted rectangle F to the input circuit of a device shown within the dotted rectangle GE (Figure 40) this device consisting in effect of the receiving amplifier G and of the switch E of Figure 1. What may be termed the switch action of the device GE is obtained by altering the bias fed to the two pushpull connected valves which constitute the first stage of the said device GE, this bias being altered in manner to be described later by potential fed over the wire Y. When the potential fed over the wire Y is such that the two said pushpull connected valves are biased to or beyond cut-ofi, the effect is as though the switch E of Figure 1 were opened, while when the potential fed over the wire Y is such that the valves are operative to amplify, the effect is as if the switch E of Figure 1 were closed It will be noted that the self capacities of these pushpull connected valves are neutralized by neutralizing condensers NC. Neutralization substantially increases the efliciency of the switching action obtained. Output from the device GE is passed through an amplifier AMP (note Figurea) to one side of the hybrid coil set E to which the subscribers line L is connected as before. The interposition of the amplifier AMP prevents any speech currents from the line L reaching the filter F leading to the apparatus (3C1. The other side of the hybrid coil set H is connected through suitable filter F to the input terminals of a device shown within the dotted rectangle DA said device constituting in efiect the switch D of Figure 1 and an amplifier. The switch action of the device DA is obtained in manner analogous to that in which it is obtained in the deviceGE by alterirx the bias upon the push-pull connected valves constituting the first stage of the said device DA, the bias potential for these valves being fed over the line X. The output from the device DA is taken to the radio transmitter (notshown). Energy from a point intermediate the amplifier AMP and the output terminals of the device GE is passed through a suitable filter F and a suitable variable attenuation device indicated within the rectangle GC1 to the input terminals of the apparatus indicated at BB.

As will be seen this apparatus consists of two single valve stages of amplification in cascade followed by a pushpull connected stage which feeds through a transformer T2 to two valves V2 arranged to act as diodes. The valves V2 are so connected together that they act as a full wave rectifier, the rectified output passing through a time control circuit consisting of a resistance R2 shunted by a condenser C2. The transformer T2 valves V2 resistance R2 and condenser C2 are analogous to the transformer T2 valve V2 resistance R2 and condenser C2 of Figure 3, the amplifier portion preceding the transformer T2 being analogous to thedevice B of Figure 3. Energy froma point intermediate the hybrid coil set H and the filter F leading to the device DA, is passed through a further suitable filter F and a variable attenuation device GC2 to the input terminals of a deviceshown within the dotted rectangle AA, this device consisting of a single valve stage of amplification followed by a pushpull connected stage whose output is fed via the transformer T1 to a pair of triodes V1 arranged to operate as a full wave rectifier and having a time control output circuit consisting of a resistance R1 shunted by a condenser C1. The transformer T1 valves V1 resistance R1 and condenser C1 are analogous to the transformer T1 valve V1 resistances R1 and condenser C1 of Figure 3 the stages of amplification within the rectangle AA preceding the transformer T1 being analogous to the device A. Tapping points upon the resistance R1 and R2 are connectedin effect to the grid and cathode respectively of the first valve of 'a two valve arrangement shown within the rectangle CV3 the apparatus within this rectangle being analogous to the valve V3 of Figure 3 plus the apparatus at C (which is not shown in Fig. 3). The cathodes of the valves V1 and V2, are all connected together and as will be obvious from Figure 4 the connections are such that differential control of the unit CV3 is obtained from the voltages set up in the resistance R1 and R2. Connection is taken from the plate of the first valve in the unit CV3 to the effective centre point of the push-pull grid circuit of the first pushpull connected ampliher of the apparatus at GE, and, similarly, connection is taken from the plate of the second valve at CV3 to the effective centre point of the pushpull grid circuit of the first push-pull connected amplifier of the apparatus at DA.

These connections, which are indicated by the references Y and X respectively, arethe connections for feeding the controlling bias potentials to the devices at GE and DA respectively so as to cause these devices to give the required switch actions. The rectangle PU encloses a potentiometer unit from which steady grid and anode potentials are tapped for the various valves as indicated.

The apparatus of Figure 4. includes another unit not included in the schematic representations of Figures 1 and 3 said unit being indicated at Z of Figure 4b. As will be seen the unit Z receives energy from the input terminals of the variable attenuation device CTCl inserted in the path to the input terminals of the device BB and the said device Z supplies control grid bias via the lead GB to the first valve of the apparatus at AA. The apparatus at Z is such as to provide when actuated, an output potential sufficient to bias the first valve of the apparatus at AA to the cut off point, the said apparaus at Z being further so designed that it is actuated by received speech level but not by received noise level so that when speech output comes from the device GE the device Z renders the device AA inoperative but mere noise from the device GE will not cause the device Z to render the device AA inoperative. The apparatus at Z forms the subject of another patent application and no claim is made in this specification to the said apparatus per se. The apparatus at Z sharply difierentiates between received noise level and received speech. level by reason of the fact that the noise level will be below the speech level and the said apparatus at Z is designed so that when the input thereto increases above a predetermined level (corresponding to the speech level to be expected) the output from the said apparatus decreases relatively sharply to a very low value. This device is preferably provided with a time control circuit whereby the time of restoration of the device may be regulated and the time of restoration should be made long to serve as a hang-over for weak syllables and echo effects Figure 4 shows also a quantity of detail apparatus well known per se, such for example as testing jacks, meters and so forth, but it is not thought necessary to describe this apparatus in the present specification since it has nothing to do with the present invention as such.

I claim:

1. A two-Way telephone or like electrical signalling system suitable for use for example as a terminal equipment in a two-way radio telephone system and comprising a normally paralyzed transmitting channel, a normally made receiving channel including a receiving amplifier, a hy brid coil set or its equivalent for associating the input terminals of the transmitting channel and the output terminals of the receiving amplifier with a subscribers line, an amplifier detector connected to receive its input at a point between said hybrid coil set and the output terminals of said receiving amplifier, a second amplifier detector whose input terminals are energized from a point between the input terminals of the receiving amplifier and the point at which the normally made receiving channel may be paralyzed, connections between said second and first mentioned amplifier detectors whereby said second mentioned amplifier detector controls the gain of the first in dependence upon the input to said second amplifier detector and a blocking device energised in dependence upon the output of the first mentioned amplifier detector for making the transmitting channel and paralyzing the receiving channel.

2. A two-way telephone or like electrical signalling system suitable for use, for example as a terminal equipment of a two-way radio telephone system and comprising a normally paralyzed transmitting channel, a normally made receiving channel including a receiving amplifier, a hybrid coil set or its equivalent for associating the input terminals or" the transmitting channel and the output terminals of the receiving amplifier with a subscribers line, an amplifier connected to receive its input at a point between said hybrid coil set and the output terminals of said receiving amplifier, a second amplifier whose input terminals are energized from a point between the input terminals of the receiving amplifier and the point at which the normally made receiving channel be paralyzed, rectifiers energized from the outputs of each of said last menticned amplifiers, a. further amplifier connected to be differentially controlled in dependence upon the voltage outputs from the two said rectifiers, and means for utilizing the output from said further amplifier to make the normally paralyzed transmitting channel and paralyze the normally made receiving channel.

3. A system as claimed in claim 2 in which the plate circuit of each rectifier includes a time control circuit.

4. A system as claimed in claim 2 and in which the output circuit of the further amplifier includes a time control circuit.

FRANCIS MONEY GRAHAM MURPHY. 

